1. Field of the Invention
This invention relates to a solar energy absorptive coating for absorbing sunlight energy, an arrangement of the coating on a substrate surface of a substrate and a method for manufacturing the arrangement. Moreover a use of the arrangement is provided.
2. Description of the Related Art
A sun energy collecting unit of a sun field power plant based on the concentrated solar power technique comprises for instance a trough with parabolic mirrors and a heat receiver tube. The heat receiver tube is arranged in a focal line of the mirrors. By the mirrors the sunlight is focused to the heat receiver tube, which is filled with a heat transfer fluid, e.g. a thermo-oil. Via the heat receiver tube the energy of the sunlight is coupled into the heat transfer fluid. Solar energy is converted to thermal energy.
In order to maximize an efficiency, with which the energy of the sunlight is coupled into the heat transfer fluid, a solar energy absorptive coating is attached on a surface of the heat receiver tube. Such an absorptive coating commonly comprises a multilayer stack with sequentially deposited thin film layers having different optical characteristics.
An essential overall optical characteristic of the absorptive coating is a high solar absorbance (low solar reflectivity) for wavelengths of solar spectrum (absorption radiation). Additionally a low emissivity (high reflectivity) for infrared radiation is advantageous.
An absorptive coating with absorbing layers having absorbing layer materials is known from U.S. Pat. No. 5,523,132. The absorbing layers comprise cermet as absorbing layer materials. The cermet is a composite material with at least two physical phases. One phase is formed by at least one composite matrix material which is a dielectric material. A second phase is formed by at least one kind of metal particles. The amount of the metal particles distributed within the composite matrix material is given by the filling factor (F.F.). An extinction coefficient of the absorbing layer material increases with the filling factor of the metal particles. As a consequence the absorbance of the absorbing layer increases with the increasing of the filling factor, too (a layer thickness of the absorbing layer is the same). Additionally these absorbing layers are transparent or nearly transparent for electromagnetic waves of the infrared region.